Graphical devices utilizing an input signal corresponding in time to the location of an object are finding increased application in such fields as facsimile transmission, computer data input devices, type compositor and layout systems, computer graphics for the preparation of engineering and architectural drawings, and handwriting verification and analysis. The application of such position-determining systems, however, has been limited by cost and performance considerations: systems with high resolution and high accuracy are so expensive as to preclude their use in many applications. Even in applications where a high degree of accuracy is not a requirement, the relative high cost of available systems may limit or prohibit their use.
There have been many varieties of position-determining systems making use, in some cases, of waves transmitted through air and, in other systems, of waves transmitted by a solid medium. For example a stylus or cursor, whose position is to be determined, may include a spark generator that produces an acoustic shockwave that is received by a pair of linear transducers positioned in fixed coordinate planes. In such a system the accuracy is limited by such factors as the finite size of the spark and the jitter or change in spark location between succesive energizations of the gap, and the variations in the speed of transmission through the air between the source and the receptor because of drafts and temperature changes.
Another arrangement makes use of an array of wires embedded in a subsurface along perpendicular coordinates. A magnetic field is generated at the location of a cursor which is picked up at the location of the nearest coordinate intersection of the embedded wires and carried along the wires to receptors located at the respective ends of the wires. Such a system is expensive to manufacture, for the wires must be permanently embedded in precise locations to avoid substantial reading errors and many wires must be provided and connected to achieve acceptable resolution.
Systems are also manufactured using such embedded wires to transmit vibrational pulses introduced magnetostrictively by a magnetic field at the cursor. With all of these constructions, the problems of cost and accuracy continue to limit the practical applications.
The present invention overcomes many of these difficulties and permits the construction of a low-cost position-determining system that is capable of high resolution and high accuracy. A series of shock-waves generated by one or more linear transducers are transmitted through the air, reflected by the object whose position is to be determined and received by the same or other transducers. The bi-directional sound path permits accurate performance independent of the presence of strong air drafts. One or more calibration reflectors are used to further minimize errors caused by variations in the propagation velocity of the waves through the air. The particular arrangement of transducers and the calibration paths are dependent upon the application and the accuracy that is required.
It is accordingly an object of the invention to provide an apparatus and method for determining the position of an object with improved accuracy and at lower cost.
It is an object also to provide such a system that is highly versatile and suitable for a wide range of applications including signature verification and computer graphics as well as many other applications.
It is an object to provide a system in which versatility and simplicity of application are enhanced by the use of a passive wireless cursor whose position is determined by the reflection of air-borne waves.
A practical system of the kind described here must also be capable of measuring with considerable accuracy the position of an object of relatively small size, such as a hand-held stylus and which may move to within a short distance from the transducer. It is thus another object to provide an improved Sell-type transducer, suitable for both transmission and reception of ultrasonic shockwaves, that has increased sensitivity and is practical in a system based on the reflection of acoustic waves from a cursor or hand-held stylus.
Another object is to provide a Sell-type transducer having improved sensitivity coupled with rapid recovery time after the transmission of an acoustic shockwave.